Abrupt global events in the Earth’s history: a physics perspective Gregory Ryskin Robert R. McCormick School of Engineering and Applied Science Northwestern University, Evanston, Illinois 60208 ABSTRACT The timeline of the Earth’s history reveals quasi-periodicity of the geological record over the last 542 Myr, on timescales close, in the order of magnitude, to 1 Myr. What is the origin of this quasi-periodicity? What is the nature of the global events that define the boundaries of the geological time scale? I propose that a single mechanism is responsible for all three types of such events: mass extinctions, geomagnetic polarity reversals, and sea-level fluctuations. The mechanism is fast, and involves a significant energy release. The mechanism is unlikely to have astronomical causes, both because of the energies involved, and because it acts quasi- periodically. It must then be sought within the Earth itself. And it must be capable of reversing the Earth’s magnetic field. The last requirement makes it incompatible with the consensus model of the origin of the geomagnetic field – the hydromagnetic dynamo operating in the Earth’s fluid core. In the second part of the paper, I show that a vast amount of seemingly unconnected geophysical and geological data can be understood in a unified way if the source of the Earth’s main magnetic field is a ~200-km-thick lithosphere, repeatedly magnetized as a result of methane-driven oceanic eruptions, which produce ocean flow capable of dynamo action. The eruptions are driven by the interplay of buoyancy forces and exsolution of dissolved gas, which accumulates in the oceanic water masses prone to stagnation and anoxia. Polarity reversals, mass extinctions, and sequence boundaries are consequences of these eruptions. Unlike the consensus model of geomagnetism, this scenario is consistent with the paleomagnetic data showing that “directional changes during a [geomagnetic polarity] reversal can be astonishingly fast, possibly occurring as a nearly instantaneous jump from one inclined dipolar state to another in the opposite hemisphere”. 1 1. Framing the questions 1.1 Introduction What makes physics different? Steven Weinberg put it well: “ One of t he primary goal s of physics is to understand the wonderful variety of nature in a unified way ” (Weinberg 1999) . By contrast, historical science s such as biology or geology focus on the particular, and deal with an overwhelming amoun t of detail , most of it contingent on the actual path of development (the history) of their subject matter. An attempt to find a unifying theme in a maze of historical detail may encounter strong resistance. But on those rare occasions when such an attempt succeeds, the result is a transformation of revolutionary proportions. Examples are the molecular biology revolution , and the plate tectonic s revolution in Earth science. With few exceptions, the Earth science community was firmly opposed to Alfred Wegen er’s proposal of continental drift for 50 years, until in 1963 Lawrence Morley, and independently Vine and Ma tthews (1963) , combin ed the sea -floor -spreading hypothesis of Hess (1962) with the geomagnetic polarity reversals (whose reality was denied for eve n longer time ). Th e Vine -Matthews -Morley hypothesis started the plate tectonics revolution; t he conversion of the Earth science community was complete in a few years (Hallam 1989 , Oreskes 2001 ). Prior to these developments , Wegener’s proposal was deemed un acceptable because “It was too large, too unifying, too ambitious. Features that were later viewed as virtues of plate tectonics were attacked as flaws of continental drift .” (Oreskes 2001, p. 11) . Some of the processes that were initially inferred on the basis of geological record are now directly measurable. For example, the theory of plate tectonics implie d that continents we re moving with velocities of the order of a few centimeters per year ; such movements can now be tracked using the global positioni ng system . Nevertheless, the most interesting questions in Earth science, and the answers to them, must be sought in the geological record. As in cosmology (another historical science), th at record is unique, and the system is not subject to experimentati on. In the case of cosmology , the assumptions of symmetr y (homogeneity and isotropy) on the large scale reduce the comp lexity of the problem enormously ; for geology, nothing comparable is possible. It is hardly surprising that , with the exception of plate tectonics, no progress has been made to ward a “unified theory” of geology . 1.2 Quasi -periodicity of the geological record One feature of the Earth’s history may present an opportunity for theoretical analysis . The geological time scale (Gradstein et al. 2004 ; see Fig. 1 ) reveal s quasi -periodicity of the geological record over the last 542 M yr , on timescales close , in the order of magnitude , to 1 Myr . For 2 example , the intervals between geomagnetic polarity reversals typically range from 0.2 to 2 Myr ; betw een geological stage boundaries – from 1 to 10 Myr , etc. Exceptions do exist, e.g., there were no polarity reversals between 84 and 124 Myr ago . The geological time scale is the final result of assimilation and interpretation of a staggering amount of geo logical and geophysical data . This is done within the conceptual framework of stratigraphy , the study of sedimentary rock strata , their temporal correlation and order ing (ICS 2009) . The very existence of stratigraphy is predicated on the presence in the ge ological record of clearly identifiable markers, reflecting some global events . Given the imperfection of the geological record, i t is obvious that abrupt , geologically instantaneous global events would leave the best possible markers. But it is far from o bvious why the Earth sh ould produce such events . The fact that it has, at least for the last 542 Myr , is remarkable; it is telling us something very important , but in order to understand the message, the right question s must be asked first . My purpose in the present paper is to frame the question s (Part 1) , and also to describe my own attempt s to provide answer s (Part 2) . Among the most important branches of stratigraphy are biostratigraphy (based on the fossil content of the rock), magnetostraigraphy (ba sed on the geomagnetic polarity recorded in the rock), and sequence stratigraphy ( based on sequences of strata deposited on continental margins by the cycles of sea -level rise and fall ). Biostratigraphy was developed in the first half of the nineteenth cen tury (Hancock 1977, Hallam 1989) , magnetostratigraphy – in 1960’s (Glen 1982) , and sequence stratigraphy – in 1970’s. All three have been spectacularly successful in practical terms . Biostratigraphy , in particular, provide s relative ages of the strata; aft er absolute ages of a number of tie points were determined using the radio metric dating, it became possible to establish the geological time scale for the last 542 Myr (the Phanerozoic eon) . The rocks older than 542 Myr contain hardly any fossils , and bios tratigraphy cannot be used . Yet, there is still no understanding of what caused the global events on which these powerful methodologies are based. This is not unusual : astronomy was put to practical use s such as the calendar and navigation long before ter restrial and celestial mechanics were unified by Isaac Newton. Some attempts at explanation have been made in the past . Georges Cuvier suggested that discontinuities in the fossil record reflected mass extinctions produc ed by environmental catastrophes , such as inundations by the sea ; he did not discuss possible causes of the catastrophes per se . His influential essay of 1812 was entitled “ Discours sur les révolutions de la surface du globe ”; an English t ranslation came out the following year (Cuvier 1813 ) and went through several editions . At that time, catastrophism had eloquent supporters in Britain , but they were soon outnumbered by the uniformitarians , whose motto was “the present is the key to 3 the past”, and who took this to mean that only the proces ses observable now may have operated in the past (Hallam 1989). Charles Darwin, in particular, denied the reality of mass extinctions altogether , and ascribed any evidence for them to gaps in the geological record (Raup 1994) . This is not surprising : even though Darwin used the fossil succession as evidence of evolution , th e theory of evolution offers no explanation for mass extinctions (Raup 1994) . 1.3 Mass extinctions and biostratigraphy In m ost of the geological literature, the designation “mass extinc tion” is reserved for the most severe extinctions in the Earth’s history, such as the Permian -Triassic ca. 251 Myr ago , the Cretaceous -Tertiary (Cretaceous -Paleogene ) ca. 65 Myr ago , and a few others . Each of t hese extinctions eliminated ~7 0 to 9 0% of the total number of species ; consequently , these extinctions define the most significant boundaries of the geological time scale . I n particular, the two extinctions mentioned above define the boundaries between the geological eras, Paleozoic, Mesozoic, and Cen ozoic, which together compris e the Phanerozoic eon . However, a s emphasized by Raup (1994), other extinctions – e.g., th e ones marking stage boundaries – were not qualitatively different from the “Big Five”, and should be included in the same category. Hall am and Wignall (1997, p. 1) define mass extinction as “an extinction of a significant proportion of the world’s biota in a geologically insignificant period of time” . Generally speaking , a biostratigraphic boundary is marked by a mass extinction : “Five pe rcent [of the total number of species] is roughly the extinction level that normally defines [the boundary of] the ‘biostratigraphic zone’ – the smallest unit in geologic time recognizable by fossils on a global or near -global basis.